1. Field of the Invention
The present invention relates to a second harmonic generation (SHG) laser stabilizing control device for stabilizing the intensity of output light of an SHG laser.
2. Description of the Related Art
As is well known, in order to increase the recording capacity of an optical disk, a short-wavelength coherent light source is required. More specifically, as the surface recording density of a disk improves, the size of a condensed light spot on the disk needs to be reduced. From the simple principle that the size of a condensed light spot is proportional to the wavelength of the light, a light source which generates short-wavelength light is essentially demanded. Thus, a small and practical short-wavelength light source is desired.
One conventional technique for realizing short-wavelength light is second harmonic generation (SHG) using a near-infrared semiconductor laser and a quasi phase match (QPM) type polarization inversion waveguide device (Yamamoto, et al., "Optics Letters" Vol. 16, No. 15, 1156 (1991)).
FIG. 20 is a schematic structural view of a conventional blue light source using a polarization inversion waveguide device. Referring to FIG. 20, the blue light source includes a 0.85 .mu.m-band 100 mw-class AlGaAs distributed Bragg reflection (DBR) semiconductor laser 1019, a collimator lens 1020 with a numerical aperture (NA) of 0.5, a half-wave (.lambda./2) plate 1021, a focusing lens 1022 with an NA of 0.5, and a polarization inversion waveguide device 1023 composed of a wavelength conversion device.
The DBR semiconductor laser 1019 includes a DBR region for controlling the oscillation wavelength. The DBR region incorporates an internal heater for changing the oscillation wavelength.
The polarization inversion waveguide device 1023 includes an LiTaO.sub.3 substrate 1024 and an optical waveguide 1025 and periodical polarization inversion regions 1026 formed on the substrate 1024.
Laser light which has been collimated by the collimator lens 1020 is incident on the .lambda./2 plate 1021, where the polarization direction of the laser light is rotated. The resultant laser light is then focused by the focusing lens 1022 on an end face of the optical waveguide 1025 of the polarization inversion waveguide device 1023. The laser light then propagates through the optical waveguide 1025 as well as the polarization inversion regions 1026. During the propagation, part of the laser light is converted into harmonic, which is output from the output end face of the optical waveguide 1025, together with the fundamental wave of the laser light which has been remained unchanged.
The phase match wavelength allowance of the polarization inversion waveguide device 1023 is set as small as about 0.1 nm to increase the efficiency of wavelength conversion. In order to meet this setting, the oscillation wavelength is set within the phase match wavelength allowance of the polarization inversion waveguide device 1023 and fixed by controlling a current applied to a DBR region of the DBR semiconductor laser 1019. With this configuration, when laser light with an intensity of 70 mW is incident on the optical waveguide 1025, about 3 mW of blue light with a wavelength of 425 nm is obtained.
The DBR semiconductor laser 1019 is essentially composed of an active region for providing a gain and the DBR region for controlling the oscillation wavelength. The active region has a diffraction lattice which makes the DBR region transmittable for the wavelength of 850 nm of the laser light. With this arrangement, the oscillation wavelength is controlled by the wavelength of the light reflected by the DBR region. The oscillation wavelength may also be changed by changing the refraction index of the DBR region. The refraction index of the DBR region may be changed by a method where a wavelength-changeable current applied to the DBR region is changed, a method where the temperature is changed using an electronic cooling element (e.g., a Peltier element), or the like.
FIG. 3 shows the relationship between the wavelength-changeable current applied to the DBR region and the oscillation wavelength of the DBR semiconductor laser. As is apparent from FIG. 3, the oscillation wavelength shifts toward a longer wavelength as the wavelength-changeable current increases by repeated mode hopping. The oscillation wavelength obtained at a certain current point when the wavelength-changeable current is gradually being increased is different from the oscillation wavelength obtained at the same current point when the wavelength-changeable current is gradually being decreased. That is, hysteresis is exhibited. Accordingly, when the oscillation wavelength is changed by changing the wavelength-changeable current, a measure against the hysteresis is required, where a wavelength-changeable current which is smaller or larger by a predetermined amount than a target wavelength-changeable current with which a desired wavelength is obtained is first set, and then the set wavelength-changeable current is gradually increased or decreased to the target wavelength-changeable current with which the desired wavelength is obtained.
The conventional SHG laser using the above-described DBR semiconductor laser as a fundamental wave light source has the following problems.
When the oscillation wavelength is changed by changing the wavelength-changeable current, a wavelength-changeable current with which the second harmonic power of output light of the SHG laser is near maximum needs to be determined as the current to be supplied to the DBR semiconductor laser.
As described above, however, the oscillation wavelength of the DBR semiconductor laser shifts by repeated mode hopping with respect to the wavelength-changeable current. Accordingly, when the wavelength-changeable current is fixed so that the second harmonic power of output light of the SHG laser is maximum, if the fixed wavelength-changeable current is near a current amount at which the mode hopping occurs, the resultant second harmonic power of the SHG output becomes unstable.
The second problem is as follows. After the wavelength-changeable current is set at a stable point, the temperature of the SHG laser is varied by a Peltier element or the like, to search for a temperature at which the second harmonic power of output light of the SHG laser is maximum. This search includes searching for an inflection point (maximal value) of the second harmonic power while changing the temperature by a predetermined step and determining a temperature corresponding to the inflection point.
However, a plurality of inflection points may be found in the second harmonic power of output light of the SHG laser when the temperature is changed. This occurs due to a variation in the characteristics of the polarization inversion waveguide device, such as a non-uniform width of the waveguide. If a plurality of inflection points exist, it is difficult to precisely recognize the maximum second harmonic power and thus to determine the temperature of the SHG laser corresponding to the maximum second harmonic power.
The method for changing the temperature of the SHG laser includes a method where temperature servo control is performed to make the temperature of the SHG laser constant, so that the temperature of the SHG laser is set at a target temperature, for example.
However, a control error and a temperature offset value due to circuit offset exist between the temperature target value and the actual detected temperature. Accordingly, even if the temperature of the SHG laser is adjusted to the temperature target value, the actual temperature of the SHG laser does not necessarily agree with the target temperature, or it takes time to approximate the actual temperature to the target temperature.
The third problem is as follows. When the wavelength-changeable current is controlled so that the second harmonic power of output light of the SHG laser is stabilized at around the maximum thereof, the output characteristics of the semiconductor laser need to be kept constant without change.
However, when the current applied to the active region of the semiconductor laser is controlled by constant voltage driving, a temperature change at the DBR region may affect the active region. This occurs because the wavelength-changeable current is changed so swiftly or the DBR region and the active region are so close to each other that the temperature of the DBR region of the semiconductor laser becomes uncontrolled although the SHG laser is under temperature control of keeping the temperature thereof constant. As a result, the output characteristics of the semiconductor laser vary.
When the temperature of the SHG laser is changed so as to search for a temperature at which the second harmonic power of output light of the SHG laser is maximum, also, temperature change occurs in the DBR region because the current applied to the active region is controlled by constant voltage driving as described above. This also results in a variation in the output characteristics of the semiconductor laser.